In a case where a pulse width of pulsed light is short (for example, in a case of 10 picoseconds or less), an SHG (second harmonic generation) autocorrelation measurement device using a nonlinear optical crystal being capable of generating second harmonic light by incidence of the pulsed light is used for measurement of the pulse width. Such autocorrelation measurement devices are described in Patent Documents 1 and 2 and Non Patent Document 1.
A conventional autocorrelation measurement device includes a Michelson interferometer having a beam splitter, a fixed reflection unit and a movable reflection unit. Incident pulsed light is branched into two components by the beam splitter to become first pulsed light and second pulsed light. The first pulsed light is reflected by the fixed reflection unit and is returned to the beam splitter, and the second pulsed light is reflected by the movable reflection unit and is returned to the beam splitter. The first pulsed light and the second pulsed light returned to the beam splitter are incident on the nonlinear optical crystal via the beam splitter.
Second harmonic light is generated in the nonlinear optical crystal on which the first pulsed light and the second pulsed light are incident, and the second harmonic light is detected by a detection unit. The movable reflection unit is movable, and its movement changes a delay time between the first pulsed light and the second pulsed light which are incident on the nonlinear optical crystal. Further, a detection result by the detection unit is changed due to the change in the delay time. Therefore, the pulse width of the incident pulsed light can be analyzed on the basis of the relationship between the delay time and the detection result by the detection unit.